1. Field of the Invention
This invention broadly relates to orthodontic appliances that are precoated with an adhesive for facilitating bonding of the appliances to tooth structure. More particularly, the present invention concerns release substrates that are releasably connected to a layer of adhesive that has been coated onto a base of orthodontic appliances.
2. Description of the Related Art
Orthodontic treatment involves movement of malpositioned teeth to orthodontically correct positions. During treatment, tiny orthodontic appliances known as brackets are connected to surfaces of the patient's teeth, and an archwire is placed in a slot of each bracket. The archwire forms a track to guide movement of the teeth to desired positions for correct occlusion. End sections of the archwire are often received in appliances known as buccal tubes that are fixed to the patient's molar teeth.
In the past, orthodontic appliances were connected to teeth by welding or brazing each bracket or buccal tube to a band that was then placed over the desired tooth in encircling relation. In more recent years, however, it has become common practice to bond orthodontic appliances directly to the surface of the tooth. Orthodontic brackets that are directly bonded to tooth surfaces provide a more aesthetic appearance than the appearance of brackets that are welded to bands, and help alleviate the problem of a "tinsel tooth" or "metallic mouth" appearance that is often associated with orthodontic treatment.
For many years, it was common practice to apply a layer of orthodontic adhesive to the base of directly-bonded appliances immediately before the appliances were placed on the tooth. In some instances, a quantity of adhesive was dispensed onto a mixing pad or dispensing well and a small spatula or other hand instrument was then used to apply a small dab of adhesive to each appliance. In other instances, a quantity of adhesive was dispensed from a syringe directly onto the base of the appliance.
Adhesive precoated brackets are also now available and represent a significant advantage to the orthodontist. Adhesive precoated brackets have a bonding base upon which the manufacturer has applied a precise quantity of adhesive such as a photocurable adhesive. When it is desired to mount the bracket on a tooth, the bracket is simply removed from the package and placed directly onto the tooth surface.
Examples of adhesive precoated brackets are described in U.S. Pat. Nos. 4,978,007, 5,015,180 and 5,328,363, all of which are assigned to the assignee of the present invention. In certain embodiments of the inventions described in those patents, the bracket and adhesive are packaged in a container that protects the adhesive from light, evaporation, oxidation, contamination, humidity and sublimation. In some of those embodiments, the coating of adhesive on the packaged bracket is in contact with a release liner or release coating that helps prevent the adhesive from being disturbed when the bracket is lifted from the package for use.
As can be appreciated, adhesive precoated brackets represent a significant time savings for the orthodontic practitioner because the adhesive need not be carefully applied to the base of each bracket before placement of the bracket onto the patient's tooth. In addition, the manufacturer can control the quantity of adhesive placed on the bracket so that there is sufficient adhesive to substantially fill the space between the bracket base and the tooth when the bracket is pushed into position, and yet there is not an inordinate amount of adhesive that might otherwise require excessive clean-up around the perimeter of the bracket base. Optionally, the adhesive is a light-curable adhesive so that the bracket can be carefully positioned in a proper orientation on the tooth surface before a curing lamp is activated to cure the adhesive and securely fix the bracket in place.
The packaged dental article described in the aforementioned U.S. Pat. No. 5,328,363 includes a dental appliance that is precoated with adhesive and located in a well of a container. In one embodiment of that patent, the adhesive rests on a flexible film made of a 0.05 mm thick sheet of fluorinated ethylene propylene copolymer ("Teflon" brand film), and the film is secured to the container at a location spaced from the appliance. As a consequence, the film tends to peel away from the adhesive when the appliance is removed from the container in order to inhibit separation of the adhesive from the appliance. Similar packaged articles using a polyester film instead of a fluorinated ethylene propylene copolymer film are also known.
In general, the adhesives used for adhesive precoated brackets that are received in a container or package having a release liner or coating are more viscous (i.e., less fluid) than other available orthodontic bonding adhesives, in part to ensure that the adhesive retains its shape and does not separate or distort when the bracket is lifted from the package for use. However, some orthodontists prefer the use of less viscous (i.e., more fluid) adhesives in order to facilitate manipulation of the bracket before the adhesive is cured. For example, brackets with less viscous adhesives are relatively easy to slide along the tooth surface when an effort is made to align the bracket in a proper, precise orientation on the tooth before the adhesive is cured.
Unfortunately, the flexible film described in the packaged article mentioned above, whether made of fluorinated ethylene propylene copolymer or polyester, is not always satisfactory for use with certain adhesives. For example, when soft, tacky, less viscous, hydrophilic adhesives are used, it has been found that a portion of the adhesive is sometimes left on the film when an attempt is made to lift the appliance from the container and detach the adhesive from the film. In those instances, there may not be a sufficient amount of adhesive remaining on the bracket to provide a bond strength adequate to retain the bracket on the tooth during the course of orthodontic treatment. Moreover, adhesives having a relatively low viscosity tend to slowly flow across the film and from the space beneath the bracket over extended periods of time, creating shelf life problems.
Additionally, when soft, tacky, less viscous adhesives are used with conventional adhesive precoated appliance packages, the shape of the adhesive may distort as the appliance is lifted from the container. In some instances, the adhesive once detached from the film may assume a configuration that is unsatisfactory for direct bonding unless additional steps are undertaken to shift the adhesive by hand back to its original, pillow-like shape. For example, when lifting the appliance from the container, some of the adhesive may be shifted to one side of the appliance base, such that the opposite side of the base does not have a satisfactory amount of adhesive for bonding. If the appliance is secured to the tooth in such a manner that a void space is present between a portion of the base and the opposed tooth surface, the void space may result in premature, spontaneous debonding of the appliance from the tooth, a nuisance that is best avoided. Furthermore, in some instances the void space can establish a pocket that receives food and debris, facilitating the formation of caries.
As can be appreciated, there is a need in the art to provide a release substrate for adhesive precoated appliances that would be satisfactory for use with a greater variety of adhesives than the release substrates that are presently known. Moreover, it would be an advantage to provide a release substrate for adhesive precoated orthodontic appliances that provides satisfactory results for use with adhesives that have a relatively low viscosity and/or are relatively tacky.